Hammer for forming an undercut fastener driving slot

Abstract

An improved hammer used in the first step of a two-step method for forming an undercut fastener driving slot in the head of a fastener, such as a screw or bolt, that results in an arcuate, recessed high-torque driving slot with undercut slot sidewalls where each sidewall has a noncontinuous sphericity interrupted by an inverted and generally triangular planar surface.

I claim: 
     
       1. A hammer for forming a fastener slot in a fastener head comprising: (a) a primary surface,   (b) longitudinally-extending convex plateau defining a longitudinal axis having spaced-apart lines of origin generally transverse to said axis and common with said primary surface between said lines of origin to an intermediate region of maximum elevation from said primary surface,   (c) said plateau having a reduced dimension transverse to said plateau longitudinal axis at said region of maximum elevation and dimensionally flaring transversely outwardly in opposite directions to a greater transverse dimension at each of said lines of origin,   (d) spherical convexities adjacent said reduced dimension extending outwardly from said primary surface to a maximum elevation above said primary surface but less than the elevation of said plateau region of maximum elevation, said spherical convexities spaced-apart by said plateau in juxtaposition therewith,   (e) concave spherical-triangle wedges extending along and adjacent to said plateau, and extending below said primary surface from said lines of origin to said spherical convexities, said wedges coupled in pairs with the paired ones of said wedges spaced-apart by said plateau in juxtaposition therewith,   (f) adjacent ones of said paired wedges merging with a juxtaposed one of said spherical convexities therebetween at a merger line,   (g) an outer edge of each of said adjacent ones of said paired wedges extending from said merger line to an associated one of said lines of origin, said outer edges of said adjacent ones of said paired wedges and said merger line developing an ellipsoidal line of elevational demarcation between said primary surface and the said paired wedges and juxtaposed spherical convexity, and   (h) a planar and generally triangular surface on each of the opposite sides of said convex plateau at said intermediate region extending between and adjacent to said maximum elevation of said intermediate region and said associated one of said convex partial spheres, said triangular surfaces interrupting the continuity of said opposite sides.   
     
     
       2. The hammer of claim 1 in which said primary surface is planar. 
     
     
       3. The hammer of claim 1 in which said spherical convexities are a pair of partial spherical convexities spaced-apart by said plateau. 
     
     
       4. The hammer of claim 1 in which said merger line between said paired wedges and said juxtaposed one of said spherical convexities is common with said primary surface. 
     
     
       5. The hammer of claim 4 in which said merger line is a tangential merger point. 
     
     
       6. The hammer of claim 1 in which each of said triangular surfaces has its apex at said maximum elevation of said intermediate region.